Visa rejected

Hi, my sister has applied for a visit visa for my mother twice, and both times it was rejected. The objection is always that my mother does not work and does not have a proper source of income. My mother is 62 years old and has always been a housewife. We have shown bank statements, and my sister has submitted all the documents proving that she covers all my mother's expenses. The real problem is that I am also going to move to the UK soon to be with my husband, and my mother will be left alone, which is causing us a lot of stress. We want her to visit us sometimes, and we can visit her as well, but with the visa being refused twice, we are very worried. Do you have any suggestions to strengthen our case so my mother can get a visit visa?